A Chemical Vapor Deposition (CVD) chamber typically includes a showerhead with a perforated or porous planar surface to dispense reactants and carrier gases in a uniform manner over a second parallel planar surface, such as a semiconductor substrate surface. A substrate may be heated to a process temperature at which precursors react resulting in a film deposited on the substrate surface. Showerhead reactors, or parallel-plate reactors, lend themselves to implementation of plasma-enhanced processes, e.g., plasma-enhanced chemical vapor deposition (PECVD). A substrate support (e.g., a pedestal) may be grounded and used as one of the electrodes. A showerhead may be used as another electrode, to which RF power is applied. In another configuration, RF power may be applied to the substrate support, while the showerhead may be grounded.
Deposition uniformity may be negatively affected by variations in process parameters, such as temperatures of the substrate and the showerhead. These variations may appear during process initiation (e.g., before reaching steady state), cleaning cycles, and idling. For example, when a reactor is initially turned on, it may take long time before the temperature of the showerhead is stabilized. The showerhead may be heated by radiation from the substrate (or the substrate pedestal) and by the plasma, which may be introduced at some point during processing causing additional temperature fluctuation. At the same time, the showerhead may loose some heat to the process gases flown through the showerhead and due to radiation from its external surfaces. Further, temperature variation may be caused by other more permanent changes in the system, such as drifts in surface emissivity of the showerhead. Variable temperature may cause substantial non-uniformity of the deposited films. Furthermore, operating a showerhead at high temperatures shortens its operating life and leads to particle contamination. For example, temperatures above 300° C. can result in rapid formation of aluminum fluoride on an aluminum showerhead surface. The aluminum fluoride tends to flake off and fall onto the substrate below.
New apparatuses and methods are needed to more precisely control showerhead temperature and to operate showerheads at lower temperatures.